Conventionally, an original image reading apparatus such as a copier uses a contact image sensor. The contact image sensor has a construction as shown in FIG. 1.
In FIG. 1, reference numeral 1 denotes a glass plate on which an original image is placed; 3, a charged-coupled device ((CCD) 1 to 6) which photoelectric-converts the original image through a cell hook lens array 2 and outputs an image signal; and 4, a CCD controller which indicates timing for outputting the image signals from the respective CCDs. The respective CCDs output the image signals in turn. The image signals are transferred through a CCD controller and an interface 5 to an image reading system.
In a tight-contact type image reading system using a contact image sensor, the distance between a photoelectric converter such as a CCD and a lens passing light is short, and the original in contact with cell hock lenses is in one-to-one correspondence with photoreception pixels of CCD. Different from generally-used compression optical system, in such one-to-one optical reading system, the length of a main scanning direction of original must be the same as that of the CCDs.
However, as the CCDs are semiconductor ICs generally manufactured from silicon wafers, the size of the CCDs is limited and CCD chips in correspondence with the size of original cannot be manufactured.
Accordingly, the contact image sensor is constructed by serially connecting CCD chips. As the CCDs respectively have a corresponding original area, the sensor can be used in the one-to-one compression optical system.
The contact image sensor having the above construction has seams between chips, and upon manufacturing of the sensor, errors of CCD chip attachment occur. The errors exist in both subscanning direction and main scanning direction as shown in FIG. 2, and shift of seam between original areas and/or pixel omission occur in a read image. The seam shift and/or pixel omission causes a shift in a fine periodical pattern such as a screened dot image, which appears as a black/white stripe in an image. Further, in a contact image sensor in which a large amount of pixel omission and/or seam shift occur, even in a general character image, a curve or the like of character is shifted and the image quality is seriously degraded.